Switching device of active display device and method of driving the switching device

ABSTRACT

Example embodiments are directed to a switching device of an active display device and a method of driving the switching device, such that electrical reliability of the active display device is improved. The switching device of the active display device includes a plurality of thin film transistors (TFTs) that are connected in series. Except for a refresh time duration during which the plurality of TFTs of the switching device are simultaneously turned ON, a positive voltage is applied to at least one of the plurality of TFTs of the switching device so that a reliability of the switching device may be improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2010-0001894, filed on Jan. 8, 2010, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND

1. Field

Example embodiments relate to a switching device of an active displaydevice and methods of driving the switching device, and moreparticularly, to a switching device of an active display device and amethod of driving the switching device, the switching device having animproved electrical reliability.

2. Description

An active display device includes a switching device that controls anoperation of each pixel of the active display device. A thin filmtransistor (TFT) is used as a switching device for a display. Forexample, one pixel of the display includes one TFT as a switchingdevice. The TFT may be, for example, a silicon-TFT, an oxide TFT, anorganic TFT, or the like according to type of a semiconductor materialused as a channel material. An oxide TFT having rapid switching speed isused as the switching device.

The switching device allows a desired voltage to charge a pixel unit bymaking a current flow in a channel of the TFT for a desired period oftime. After the desired period of time, the switching device maintainsthe pixel unit in a charged state by turning off the TFT. In an activematrix liquid crystal display (AMLCD), a period of time in which the TFTis ON is determined according to a driving frequency and a resolution ofthe AMLCD. In the case of a driving operation at 120 Hz with a fullhigh-definition (HD) class, a period of time in which one TFT is an onstate is about 1/120/1080=7.7 μs. Then, for a remainder of one period(1/120=8.3 ms), the TFT remains in an OFF state. Thus, with respect toone period, the TFT of the active display device is mostly in an OFFstate.

An amorphous silicon TFT or an oxide semiconductor TFT exhibits ann-type semiconductor characteristic, and thus, in order to turn off sucha TFT, a negative gate voltage is applied to the TFT. Thus, the negativegate voltage is continuously applied to the TFT while the TFT is in theOFF state in the active display device. However, when the negative gatevoltage is continuously applied to the TFT for a desired time period, athreshold voltage may shift toward a negative voltage. As a result, aleakage current may increase as the negative gate voltage is applied tothe TFT. The shift of the threshold voltage may be severe when light isincident on the switching device. When the leakage current increases,image quality of the active display device may deteriorate.

SUMMARY

According to example embodiments, a switching device of an activedisplay device includes at least two thin film transistors (TFTs)connected in series; and at least two gate lines respectively connectedto the at least two TFTs.

According to example embodiments, each of the at least two TFTs includesan oxide semiconductor transistor having an oxide semiconductor as achannel.

According to example embodiments, the oxide semiconductor includes anoxide material selected from a group consisting of Zn-oxide, Ga—In—Znoxide, In—Zn-oxide, In—Sn-oxide, and Sn-oxide, or includes an oxidematerial obtained by doping an element selected from a group consistingof aluminum (Al), nickel (Ni), copper (Cu), tantalum (Ta), titanium(Ti), and hafnium (Hf) to the oxide material.

According to example embodiments, a number of the at least two gatelines in each pixel row of the active display device is equal to anumber of the at least two TFTs, and the at least two gate lines arerespectively connected to gates of the at least two TFTs.

According to example embodiments, a positive gate voltage is applied toeach of the at least two gate lines during a pixel charging time, thepositive gate voltage turning on all of the at least two TFTs.

According to example embodiments, during times other than the pixelcharging time, at least one of the at least two TFTs is off whileremaining ones of the at least two TFTs are on.

According to example embodiments, the at least two gate lines correspondto pixel rows of the active display device, and the at least two TFTsare respectively connected to at least two gate lines from differentpixel rows.

According to example embodiments, a positive gate voltage is applied toeach of the at least two gate lines to turn on all of the at least twoTFTs during a pixel charging time, and during times other than the pixelcharging time, at least one of the at least two TFTs is off whileremaining ones of the at least two TFTs are on.

According to example embodiments, each of the at least two TFTs includesan oxide semiconductor transistor having an oxide semiconductor as achannel.

According to example embodiments, the oxide semiconductor includes anoxide material selected from a group consisting of Zinc oxide, Ga—In—Znoxide, In—Zn-oxide, In—Sn-oxide, and Tin-oxide, or includes an oxidematerial obtained by doping an element selected from a group consistingof aluminum (Al), nickel (Ni), copper (Cu), tantalum (Ta), titanium(Ti), and hafnium (Hf) to the oxide material.

According to example embodiments, a switching device of an activedisplay device includes a double-gate thin film transistor (TFT) havingtwo gates; and at least two gate lines respectively connected to the twogates of the double-gate TFT.

According to example embodiments, the at least two gate lines correspondto each of pixel rows of the active display device.

According to example embodiments, the at least two gate lines correspondto each of pixel rows of the active display device, and the two gates ofthe double-gate TFT are connected to the at least two gates lines fromdifferent pixel rows.

According to example embodiments, a positive gate voltage is applied tothe two gates during a pixel charging time, and during times other thanthe pixel charging time, a negative gate voltage is applied to one ofthe two gates while a positive gate voltage is applied to the other oneof the two gates.

According to example embodiments, a method of driving a switching deviceof an active display device includes applying a positive gate voltageduring a pixel charging time to the at least two gate lines respectivelyconnected to at least two thin film transistors (TFTs) connected inseries such that the at least two TFTs are turned on; and, during timesother than the pixel charging time, turning off at least one of the atleast two TFTs, and turning on remaining ones of the at least two TFTs.

According to example embodiments, the method further includes applying anegative gate voltage to at least one of the at least two gate linesduring the times other than the pixel charging time.

According to example embodiments, the method further includes applyingthe positive gate voltage at least once to each of the at least two gatelines during the times other than the pixel charging time.

According to example embodiments, a duty ratio of the positive gatevoltage applied to each of the at least two TFTs is within about 0.1% toabout 10%.

According to example embodiments, a method of driving a switching deviceof an active display device includes applying a positive gate voltage totwo gates of a double-gate thin film transistor (TFT) during a pixelcharging time; and during times other than the pixel charging time,applying a negative gate voltage to one of the two gates of thedouble-gate TFT while applying the positive gate voltage to the otherone of the two gates.

According to example embodiments, the method further includes applyingthe positive gate voltage at least once to each of the two gates duringthe times other than the pixel charging time.

According to example embodiments, the method further includes applyingthe positive voltage having a duty ratio from about 0.1% to about 10% toeach of the two gates of the double-gate TFT.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages will become more apparent bydescribing in detail example embodiments with reference to the attacheddrawings. The accompanying drawings are intended to depict exampleembodiments and should not be interpreted to limit the intended scope ofthe claims. The accompanying drawings are not to be considered as drawnto scale unless explicitly noted.

FIG. 1 illustrates a structure of a switching device of an activedisplay device according to example embodiments;

FIG. 2 is a timing diagram illustrating a method of driving theswitching device of FIG. 1, according to example embodiments;

FIG. 3 is a graph obtained by measuring threshold voltage change ratiosfor different duty ratios of a positive voltage;

FIG. 4 illustrates a structure of a switching device according toexample embodiments;

FIG. 5 is a timing diagram illustrating a method of driving theswitching device of FIG. 4, according to example embodiments;

FIG. 6 is a timing diagram illustrating another example method ofdriving the switching device of FIG. 4;

FIG. 7 illustrates a voltage transfer characteristic with respect tomagnitude of a voltage applied to a top gate in a double-gate structure;

FIG. 8 illustrates a structure of a switching device having adouble-gate thin film transistor (TFT), according to exampleembodiments; and

FIG. 9 illustrates another switching device having a double-gate TFT,according to example embodiments.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Detailed example embodiments are disclosed herein. However, specificstructural and functional details disclosed herein are merelyrepresentative for purposes of describing example embodiments. Exampleembodiments may, however, be embodied in many alternate forms and shouldnot be construed as limited to only the embodiments set forth herein.

Accordingly, while example embodiments are capable of variousmodifications and alternative forms, embodiments thereof are shown byway of example in the drawings and will herein be described in detail.It should be understood, however, that there is no intent to limitexample embodiments to the particular forms disclosed, but to thecontrary, example embodiments are to cover all modifications,equivalents, and alternatives falling within the scope of exampleembodiments. Like numbers refer to like elements throughout thedescription of the figures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it may be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between”, “adjacent” versus “directlyadjacent”, etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises”, “comprising,”, “includes” and/or “including”, when usedherein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

FIG. 1 illustrates a structure of a switching device 10 of an activedisplay device according to example embodiments. Referring to FIG. 1,the switching device 10 that may control an operation of a pixel 20 mayinclude a plurality of thin film transistors (TFTs) TFT₁, TFT₂, . . . ,TFT_(m) that are connected in series, and a plurality of gate lines GL₁,GL₂, . . . , GL_(m) that are independently connected to the plurality ofTFTs TFT₁, TFT₂, . . . , TFT_(m), respectively. Each of the plurality ofTFTs, TFT₁, TFT₂, . . . , TFT_(m), may be, for example, an oxidesemiconductor transistor using an oxide semiconductor as a channelmaterial. For example, the oxide semiconductor may include an oxideselected from the group consisting of Zn-oxide, Ga—In—Zn oxide,In—Zn-oxide, In—Sn-oxide, and Sn-oxide, or may include a materialobtained by doping an element selected from the group consisting ofaluminum (Al), nickel (Ni), copper (Cu), tantalum (Ta), titanium (Ti),and hafnium (Hf) to the oxide.

As illustrated in FIG. 1, the pixel 20 is connected to the last TFTTFT_(m) from among the plurality of TFTs TFT₁, TFT₂, . . . , TFT_(m).Also, a voltage line Vd is connected to the first TFT₁ from among theplurality of TFTs TFT₁, TFT₂, . . . , TFT_(m). The plurality of gatelines GL₁, GL₂, . . . , GL_(m) are connected to corresponding gates ofthe plurality of TFTs TFT₁, TFT₂, . . . , TFT_(m), respectively. Forexample, the first gate line GL₁ is connected to the gate of the firstTFT TFT₁.

In this structure, a charging time during which a data voltage isapplied to the pixel 20 may occur only when the plurality of TFTs TFT₁,TFT₂, . . . , TFT_(m) are all in ON states. When the pixel 20 is chargedand any one of the plurality of TFTs TFT₁, TFT₂, . . . , TFT_(m) isturned off, a data voltage is not supplied to the pixel 20 and thus thepixel 20 maintains a charged state. Thus, it is not necessary that allof the plurality of TFTs TFT₁, TFT₂, . . . , TFT_(m) be turned offduring a non-charging time period, and an ON voltage may be applied toonly some, but not all, of the plurality of TFTs TFT₁, TFT₂, . . . ,TFT_(m). By doing so, it is possible to prevent that the thresholdvoltage is shifted toward the negative voltage in each of the pluralityof TFTs TFT₁, TFT₂, . . . , TFT_(m).

FIG. 2 is a timing diagram illustrating a method of driving theswitching device 10 of FIG. 1, according to example embodiments. In FIG.2, it is assumed that the switching device 10 has first through thirdTFTs that are connected in series. Thus, FIG. 2 illustrates waveforms ofgate voltages that are input to three gate lines GL₁, GL₂, and GL₃,respectively. However, in designing the switching device 10, it ispossible to use only two TFTs, or to use four or more TFTs. Referringback to FIG. 2, positive gate voltages are simultaneously applied to thethree gate lines GL₁, GL₂, and GL₃ in a time t₁. Here, the three TFTsare all turned ON so that a data voltage is applied to the pixel 20. Inother times t₂ and t₃, a negative gate voltage is applied to at leastone of the three gate lines GL₁, GL₂, and GL₃, and, as a result, atleast one of the three TFTs is turned OFF. In this regard, an effectivegate charging time during which the pixel 20 connected to the switchingdevice 10 is charged is shown in a lower part of FIG. 2. As shown, nodata voltage is applied to the pixel 20 during times other than the timet1, which is a pixel charging time in which the three TFTs are allturned ON.

During the times other than the pixel charging time t1, the positivegate voltage is separately applied to each of the three gate lines GL₁,GL₂, and GL₃ so as to separately turn on the three TFTs. For example,referring to FIG. 2, the first TFT and the third TFT are turned onduring the time t₃, the second TFT is turned on during a time t₄, andthe third TFT is turned on during a time t₅. Although only the time t₁through a time t₆ are illustrated in FIG. 2, the number of time periodsmay be greater than or less than the illustrated number of time periods.For example, times t₁ through t₁₀₈₀ may be used when 1080 pixel rows ofa full-HD class screen are scanned. In this case, for example, it ispossible to turn ON each of the three TFTs one or more times while atleast one TFT from among the three TFTs is turned OFF during any oftimes t₂ through t₁₀₈₀. During a pixel charging time t₁ in which a datavoltage is applied to the pixel 20 of one row from among the 1080 pixelrows, all three TFTs are turned on. By doing so, a threshold voltagethat may have shifted in a negative voltage direction due to applicationof a negative gate voltage may return to its original state(hereinafter, returning a threshold voltage that has been shifted towardthe negative voltage to its original state is referred to as refreshingthe TFT).

FIG. 3 is a graph obtained by measuring threshold voltage change ratioswith respect to time for different duty ratios of a positive voltagepulse having a period corresponding to a frequency of 60 Hz. Here, theduty ratio indicates a ratio of a time during which a positive pulsevoltage is applied to an entire time, wherein the entire time indicatesone period pulse. Referring to the graph of FIG. 3, as the duty ratio isincreased, a shift of a threshold voltage in a direction from a positivevoltage toward a negative voltage is reduced. Also, with respect to dutyratios equal to or greater than about 10%, the threshold voltage isshifted from a negative voltage toward a positive voltage. Thus, whileone screen is scanned in the active display device, it is possible tominimize the shift of the threshold voltage by adjusting the duty ratioof the pulse applied to each TFT. Only when a data voltage is applied tothe pixel 20 are all TFTs of the switching device 10 connected to thepixel 20 turned on, and in other times, each of the TFTs may beseparately turned on or off so as to control a shift of a thresholdvoltage. For example, the duty ratio of a positive gate voltage pulseapplied to gates of the TFTs may be within about 0.1% to about 10%.

The switching device 10 operates in the following manner. In general,when light and a gate-OFF voltage (about −8V) are simultaneously appliedto a TFT, holes that are generated in a channel layer of the TFT due tothe light move to an interface between a gate insulating layer and asemiconductor channel layer, and then are trapped. The trapped holesshift a threshold voltage of the TFT in a direction from a positivevoltage toward a negative voltage. Conversely, when a gate-ON voltage isapplied to a gate of the TFT, the holes are freed from the interfacebetween the channel layer and the gate insulating layer, and recombinedwith electrons, and instead, electrons are trapped in the interface. Inthis case, the threshold voltage is shifted in a direction from anegative voltage toward a positive voltage. In the case where only oneTFT is used as a switching device, a positive gate voltage is applied toa gate of the TFT for a relatively short amount of time, and a negativegate voltage is applied to the gate of the TFT for a relatively longamount of time. As a result, a threshold voltage of the TFT may shift ina direction from a positive voltage toward a negative voltage. In thecase of the switching device 10 of FIG. 1, at least two TFTs are used sothat it is possible to increase a time during which a positive gatevoltage is applied to each gate of the two TFTs. By doing so, it ispossible to prevent threshold voltages of the two TFTs from beingshifted in a direction from a positive voltage toward a negativevoltage, so that electrical reliability of the switching device 10 maybe improved, and lifetime of the active display device may increase.

FIG. 4 is a diagram for illustrating a structure of a switching device30 according to example embodiments. The switching device 30 of FIG. 4is similar to the switching device 10 of FIG. 1 in that the switchingdevice 30 has a plurality of TFTs TFT₁ and TFT₂ that are connected inseries, and different gate lines are connected to the plurality of TFTsTFT₁ and TFT₂, respectively. However, unlike the switching device 10 ofFIG. 1, the switching device 30 of FIG. 4 does not have separate gatelines for each transistor but allows transistors from different pixelrows to share a gate line.

In the case where the switching device 10 is configured in a mannerillustrated in FIG. 1, one active display device uses a number of uniquegate lines equal to the number of TFTs in the switching device 10 ineach of pixel rows. Thus, if the switching device 10 for one pixel 20has m TFTs, separate 1080×m gate lines are necessary for a full-HDscreen having 1080 pixel rows. On the other hand, in the case where theswitching device 30 is configured in a manner illustrated in FIG. 4, itis not necessary to add unique gate lines by using existing gate lines.

Referring to FIG. 4, the switching device 30 has two TFTs TFT₁ and TFT₂.For example, in an i^(th) pixel row, a pixel 20 _(i) is connected to thesecond TFT₂ of the two TFTs TFT₁ and TFT₂ connected in series. A voltageline Vd is connected to the first TFT TFT₁. Here, a gate of the secondTFT₂ is connected to a gate line GL_(i) of the i^(th) pixel row whereasa gate of the first TFT TFT₁ is connected to a gate line of anotherpixel row. For example, as illustrated in FIG. 4, the gate of the firstTFT TFT₁ may be connected to a gate line GL_((i−2)) of an (i−2)^(th)pixel row. In an example embodiment, the gate of the second TFT₂ may beconnected to the gate line GL_((i−2)) of the (i−2)^(th) pixel row, andthe gate of the first TFT TFT₁ may be connected to the gate line GL_(i)of the i^(th) pixel row. Also, in designing the switching device 30, thegate of the first TFT TFT₁ may be connected to another gate line such asa gate line GL_((i−1)) of an (i−1)^(th) pixel row or a gate lineGL_((i−3)) of an (i−3)^(th) pixel row.

FIG. 5 is a timing diagram illustrating a method of driving theswitching device 30 of FIG. 4, according to example embodiments.Referring to FIG. 5, a positive pulse is applied to the gate lineGL_((i−2)) in a time t₁ and a time t₃, a positive pulse is applied tothe gate line GL_((i−1)) in a time t₂ and a time t₄, and positive pulseis applied to the gate line GL_(i) in the time t₃ and a time t₅. Thus,first and second TFTs TFT₁ and TFT₂ are all turned on in the time t₃,and at this time, a data voltage is applied to the i^(th) pixel 20 _(i).A positive gate voltage is applied to the first TFT TFT₁ in the time t₁so that the first TFT TFT₁ is refreshed. Also, the second TFT₂ may berefreshed in the time t₅. In FIG. 5, the positive gate voltage isapplied only once to each of the first TFT TFT₁ and the second TFT₂during times other than the time t₃. However, provided that the firstTFT TFT₁ and the second TFT₂ are not simultaneously turned on duringother times except for the time t₃, which is a pixel charging time, thepositive gate voltage may be applied at least twice to each of the firstTFT TFT₁ and the second TFT₂.

In FIG. 4, the switching device 30 has the two TFTs TFT₁ and TFT₂.However, in other example embodiments, the switching device 30 may havethree or more TFTs. If the switching device 30 has three TFTs, the thirdTFT may be connected to a gate line GL_((i−4)) of an (i−4)^(th) pixelrow, or to a gate line GL_((i+2)) of an (i+2)^(th) pixel row. In thecase where the third TFT may be connected to the gate line GL_((i−4)) ofthe (i−4)^(th) pixel row, a gate voltage pulse is applied to each pixelrow as illustrated in FIG. 6. In the case of FIG. 6, the i^(th) pixel 20_(i) is applied a data voltage in a time t₅.

Meanwhile, the switching device 10 of FIG. 1 and the switching device 30of FIG. 4 use a plurality of TFTs. However, the switching device 10 ofFIG. 1 and the switching device 30 of FIG. 4 may use a double-gate TFThaving two gates. The double-gate TFT has a structure in which a firstinsulating layer and a first gate are arranged below a channel formed ofa semiconductor, and a second insulating layer and a second gate arearranged above the oxide semiconductor. For ease of description, thefirst gate below the oxide semiconductor is referred to as a bottomgate, and the second gate above the oxide semiconductor is referred toas a top gate.

With respect to the TFT having a double-gate structure, a transfer curveshown in FIG. 7 may be obtained by applying a fixed direct-current (DC)to an electrode of the top gate, and by applying a range of voltagesfrom about −30V to about +40V to an electrode of the bottom gate.Referring to FIG. 7, as a DC voltage increases from a negative voltageto a positive voltage at the electrode of the top gate, a thresholdvoltage of the TFT in the bottom gate increases from a negative voltagetoward a positive voltage. For example, in order to turn on the TFTwhile a voltage of about −10V is applied to the top gate, a voltage ofabout +15V or a larger voltage is applied to the bottom gate.

It is possible to configure a switching device by using a characteristicof the double-gate TFT. For example, FIG. 8 is a diagram of a switchingdevice 10′ having a structure in which separate first and second gatelines GL₁ and GL₂ are respectively connected to first and second gatesG₁ and G₂ of a double-gate TFT. For example, the first gate G₁ isconnected to the first gate line GL₁, and the second gate G₂ isconnected to the second gate line GL₂. Thus, the switching device 10′ ofFIG. 8 is obtained by replacing the plurality of TFTs in the structureof the switching device 10 of FIG. 1 with the double-gate TFT. Accordingto the structure of the switching device 10′, when a positive gateelectrode is applied to the two gates G₁ and G₂ during a pixel chargingtime, the double-gate TFT is turned on. During other times except forthe pixel charging time, a negative voltage is applied to one of thegate lines GL₁ and GL₂, wherein the negative voltage is sufficient toturn off the double-gate TFT. Referring to the graph of FIG. 7, an OFFvoltage of the TFT may be about −10V, and an ON voltage may be a voltageof about +15V. During other times, except for the pixel charging timeduring which the gates G₁ and G₂ of the double-gate TFT aresimultaneously turned on, the double-gate TFT may be refreshed in amanner described with reference to FIGS. 1 and 2.

In addition, FIG. 9 is a diagram of a switching device 30′ having adouble-gate TFT structure. The switching device 30′ of FIG. 9 isobtained by replacing the plurality of TFTs in the structure of theswitching device 30 of FIG. 4 with double-gate TFTs. Thus, the switchingdevice 30′ of FIG. 9 does not have unique gate lines for each transistorbut uses gate lines of adjacent pixel rows. For example, in a TFT at ani^(th) pixel row, a first gate G₁ may be connected to a gate lineGL_((i−2)) of an (i−2)^(th) pixel row and a second gate G₂ may beconnected to a gate line GL_(i) of the i^(th) pixel row. Although, FIG.9 illustrates that the first gate G₁ is connected to the gate lineGL_((i−2)) of the (i−2)^(th) pixel row, the first gate G₁ may beconnected to any gate line of any pixel row, except for the gate lineG_(i), of the i_(th) pixel row. An operation of the switching device 30′is somewhat similar to the operation of the switching device 30 of FIG.4.

Example embodiments having thus been described, it will be obvious thatthe same may be varied in many ways. Such variations are not to beregarded as a departure from the intended spirit and scope of exampleembodiments, and all such modifications as would be obvious to oneskilled in the art are intended to be included within the scope of thefollowing claims.

What is claimed is:
 1. A switching device comprising: at least two thinfilm transistors (TFTs) connected in series; and at least two gate linesrespectively connected to the at least two TFTs, wherein a number of theat least two gate lines in each pixel row of an active display device isequal to a number of the at least two TFTs, the at least two gate linesbeing connected to TFTs in only one pixel row, wherein, the switchingdevice is configured to, during a pixel charging time, apply a positivegate voltage to at least two gate lines such that the at least two TFTsare turned on, the pixel charging time being a time when a data voltageis applied to a pixel associated with the at least two TFTs, and theswitching device is configured to, during times other than the pixelcharging time, turn off at least one of the at least two TFTs and turnon remaining ones of the at least two TFTs.
 2. The switching device ofclaim 1, wherein each of the at least two TFTs includes an oxidesemiconductor transistor having an oxide semiconductor as a channel. 3.The switching device of claim 2, wherein the oxide semiconductorincludes an oxide material selected from a group consisting of Zn-oxide,Ga—In—Zn oxide, In—Zn-oxide, In—Sn-oxide, and Sn-oxide, or includes anoxide material obtained by doping an element selected from a groupconsisting of aluminum (Al), nickel (Ni), copper (Cu), tantalum (Ta),titanium (Ti), and hafnium (Hf) to the oxide material.
 4. The switchingdevice of claim 2, wherein the at least two gate lines are respectivelyconnected to gates of the at least two TFTs.
 5. A switching device of anactive display device, the switching device comprising: a double-gatethin film transistor (TFT) having two gates; and at least two gate linesrespectively connected to the two gates of the double-gate TFT, whereinthe at least two gate lines are in a same pixel row of the activedisplay device as the double gate TFT and the at least two gate linesare connected to the double-gate TFT in only one pixel row.
 6. Theswitching device. of claim 5, wherein a positive gate voltage is appliedto the two gates during a pixel charging time, and during times otherthan the pixel charging time, a negative gate voltage is applied to oneof the two gates while a positive gate voltage is applied to the otherone of the two gates.
 7. A method of driving a switching device of anactive display device, the method comprising: applying a positive gatevoltage during a pixel charging time to at least two gate linesrespectively connected to at least two thin film transistors (TFTs)connected in series such that the at least two TFTs are turned on; andduring times other than the pixel charging time, turning off at leastone of the at least two TFTs, and turning on remaining ones of the atleast two TFTs, the pixel charging time being a time when a data voltageis applied to a pixel associated with the at least two TFTs.
 8. A methodof driving a switching device of an active display device, comprising:applying a positive gate voltage during a pixel charging time to atleast two gate lines respectively connected to at least two thin filmtransistors (TFTs) connected in series such that the at least two TFTsare turned on; during times other than the pixel charging time, turningoff at least one of the at least two TFTs, and turning on remaining onesof the at least two TFTs; and applying a negative gate voltage to atleast one of the at least two gate lines during the times other than thepixel charging time.
 9. The method of claim 8, further comprising:applying the positive gate voltage at least once to each of the at leasttwo gate lines during the times other than the pixel charging time. 10.The method of claim 9, wherein a duty ratio of the positive gate voltageapplied to each of the at least two TFTs is within about 0.1% to about10%.
 11. A method of driving a switching device of an active displaydevice, the method comprising: applying a positive gate voltage to twogates of a double-gate thin film transistor (TFT) during a pixelcharging time; and during times other than the pixel charging time,applying a negative gate voltage to one of the two gates of thedouble-gate TFT while applying the positive gate voltage to the otherone of the two gates.
 12. The method of claim 11, further comprising:applying the positive gate voltage at least once to each of the twogates during the times other than the pixel charging time.
 13. Themethod of claim 12, further comprising: applying the positive voltagehaving a duty ratio from about 0.1% to about 10% to each of the twogates of the double-gate TFT.
 14. The switching device of claim 1,wherein the pixel is directly connected to only one of the at least twoTFTs.